Post on 29-Dec-2015
Chapter 5Java Servlets
Objectives
• Explain the nature of a servlet and its operation• Use the appropriate servlet methods in a web
application• Code the extraction of environment entries
within a servlet• Handle HTML forms within a servlet• Explain the significance of web application state• Explain the purpose and operation of HTTP
cookies and their role in state management
Java Servlets
• A servlet is a Java program that is invoked by a web server in response to a request
Client Server Platform
WebServer
Web Application
Servlet
Java Servlets
• Together with web pages and other components, servlets constitute part of a web application
• Servlets can– create dynamic (HTML) content in response to a
request– handle user input, such as from HTML forms– access databases, files, and other system resources– perform any computation required by an application
Java Servlets
• Servlets are hosted by a servlet container, such as Apache Tomcat*
*Apache Tomcat can be both a web server and a servlet container
Server Platform
WebServer
ServletContainer
The servlet containerprovides a JavaVirtual Machine for servlet execution
The web serverhandles the HTTPtransaction details
Servlet Operation
Servlet Methods
• Servlets have three principal methods
.init()invoked once, when the servlet is loaded by the servlet container (upon the first client request)
.service(HttpServletRequest req, HttpServletResponse res)
invoked for each HTTP requestparameters encapsulate the HTTP request and response
.destroy()
invoked when the servlet is unloaded (when the servlet container is shut down)
Servlet Methods
• The default .service() method simply invokes method-specific methods– depending upon the HTTP request method
.service().doGet().doPost().doHead()… etc.
Servlet Example
• This servlet will say "Hello!" (in HTML)
package servlet;import javax.servlet.http.*;
public class HelloServlet extends HttpServlet { public void service(HttpServletRequest req, HttpServletResponse res) throws IOException { PrintWriter htmlOut = res.getWriter(); res.setContentType("text/html"); htmlOut.println("<html><head><title>" + "Servlet Example Output</title></head><body>" + "<p>Hello!</p>" + "</body></html>"); htmlOut.close(); }}
Servlet Configuration
• The web application configuration file, web.xml, identifies servlets and defines a mapping from requests to servlets
<servlet> <servlet-name>HelloServlet</servlet-name> <servlet-class>servlet.HelloServlet</servlet-class></servlet><servlet-mapping> <servlet-name>HelloServlet</servlet-name> <url-pattern>/hello</url-pattern></servlet-mapping>
An identifying name for the servlet (appears twice)
The servlet's packageand class names
The pathname used to invoke the servlet(relative to the web application URL)
Environment Entries
• Servlets can obtain configuration information at run-time from the configuration file (web.xml)– a file name, a database password, etc.
• in web.xml:
<env-entry-description>password</env-entry-description><env-entry> <env-entry-name>UserId</env-entry-name><env-entry-value>Xy87!fx9*</env-entry-value> <env-entry-type>java.lang.String</env-entry-type></env-entry>
Environment Entries
• in the init() method of the servlet:
try { Context envCtx = (Context)
(new InitialContext()).lookup("java:comp/env"); password = (String) envCtx.lookup("password");} catch (NamingException e) { e.printStackTrace();} catch (ClassNotFoundException e) { e.printStackTrace();}
Handling HTML Forms
• An HTML form can be sent to a servlet for processing
• The action attribute of the form must match the servlet URL mapping
<servlet-mapping> <servlet-name>HelloServlet</servlet-name> <url-pattern>/hello</url-pattern></servlet-mapping>
<form method="post" action="hello" />
Simple Form Servlet
<form action="hello" method="post" > <p>User Id:<input type="text" name="userid" /></p> <p><input type="submit" value="Say Hello" /></p></form>
public class HelloServlet extends HttpServlet { public void doPost(HttpServletRequest req, HttpServletResponse res) throws IOException { PrintWriter out = res.getWriter(); res.setContentType("text/html"); String userId = req.getParameter("userid"); out.println("<html><head><title>Hello</title></head>" + "<body><p>Hello, " + userId + "!</p></body></html>"); out.close(); }
State Management
• session: a series of transaction between user and application
• session state: the short-term memory that the application needs in order to maintain the session– e.g., shopping cart, user-id
• cookie: a small file stored by the client at the instruction of the server
Cookies
• The Set-Cookie: header in an HTTP response instructs the client to store a cookieSet-Cookie: SESSIONID=B6E98A; Path=/slms; Secure
• After a cookie is created, it is returned to the server in subsequent requests as an HTTP request Cookie: headerCookie: SESSIONID=B6E98A
Cookie Attributes
• Name: the unique name associated with the cookie
• Content: value stored in the cookie• Expiration Date: cookie lifetime• Domain: Defines the hosts to which the cookie
should be returned• Path: Defines the resource requests with which
the cookie should be returned• Secure: if true, cookie is returned only with
HTTPS requests
Cookie Example
• Name: session-id• Content: 104-1898635-929144• Expiration Date: Monday, June 29, 2009 3:33:30 PM
• Domain: .ehsl.org• Path: /slms• Secure: no
• This cookie will be returned with all requests matching *.ehsl.org/slms*, through the indicated expiration date
Session Management
• HTTP is inherently stateless, i.e., there is no memory between transactions
• Applications must maintain a session memory if it is required
• Cookies are used to identify sessions, by recording a unique session-id
State Management
Client
Cookie[session-id]
Server
Session Memory
session
• At the start of a new session, the server sets a newcookie containing the session-id • With each transaction, the client sends the session-id,allowing the server to retrieve the session
Session Attributes
• The methodssession.setAttribute(key, value)session.getAttribute(key)
store and retrieve session memory
• key is a string; value can be any object
• For example,session.setAttribute("userid", userId);
String userId = (String)session.getAttribute("userid");
Review
• Java servlets
• Servlet methods and operation
• HTML forms and servlets
• HTTP cookies
• Web application state management